He who wishes to decide whether man is the modified descendant of some pre-existing form, would probably first enquire whether man varies, however slightly, in bodily structure and in mental faculties; and if so, whether the variations are transmitted to his offspring in accordance with the laws which prevail with the lower animals.

The Descent of Man and Selection in Relation to Sex, Chapter I (p. 5), D. Appleton & Co. 1909